A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate.
A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon substrate).
Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scamers, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Typically, a plurality of patterned layers needs to be formed on top of each other to obtain a micro-electronic integrated circuit.
Alignment of the patterned layers relative to each other should be sufficiently accurate to ensure that features in subsequent patterned layers substantially fit onto (overlay) each other as designed.
Since the minimum feature sizes may be less than 100 nm, the overlay error (from one layer to a next layer) should be less than this minimal feature size.
To this end, the alignment of a substrate to a mask should be sufficiently accurate to obtain an exposure of the substrate to a pattern of the mask within the limits set by the design rules of the integrated circuit.
When only a single first lithographic apparatus is used for definition of each of the patterned layers, the alignment procedure as described above can provide for a sufficiently accurate alignment of all patterned layers. Only a suitable layer-to-layer alignment procedure is needed for minimizing the overlay error.
However, when using (at least) a second lithographic apparatus for next patterned layer, the overlay can be influenced by differences of substrate (chuck) alignment which may vary from one lithographic apparatus to another. Thus, a large machine-related overlay error may occur since an exposure field in the second lithographic apparatus may be shifted (and/or rotated) relative to the position of the field as used in the first lithographic apparatus.
Typically, the overlay error must be determined by using an off-line tool. The off-line tool determines the machine-related overlay error from overlay marks that are present in both the patterned layer exposed on the first lithographic apparatus, and the patterned layer exposed on the second lithographic apparatus. Such overlay marks are also known to persons skilled in the art as ‘box-in-box’ targets. Such an off-line analysis is troublesome since some time is needed for the off-line measurements. After a first substrate of a batch has been exposed in the second lithographic apparatus, the following substrates can only be exposed in the second lithographic apparatus after the overlay error on the first substrate is known and, the overlay error is compensated on the second lithographic apparatus. Thus, a batch can only be processed further after the overlay error has been determined (off-line). More advanced systems use historical data to determine expected overlay error and, hence, compensation.
Moreover, the accuracy of the overlay error is not high and only improves when the number of substrates being tested is increased. Simultaneously, the time needed for off-line inspection increases even further.